1. Technical Field
This invention relates to methods for determining flow parameters of a fluid flow within a pipe using normal incident ultrasonic signals for correlation based flow measurement in general, and to methods for quantifying modulating portions of normal incident ultrasonic signals propagating across the diameter of a pipe in particular.
2. Background Information
Normal incident ultrasonic flow meters operate on the principle that flow within a pipe interacts with the propagation of normal incident ultrasonic beams traversing the pipe. Transit time, defined as the time required for an ultrasonic beam to propagate across the diameter of the pipe, can be measured using a radially aligned ultrasonic transmitter (Tx) and receiver (Rx). For an “ideal flow in a pipe” (e.g., a homogenous fluid with no transverse velocity components flowing in an infinitely rigid tube), the transit time may be given by the following relation:t=D/Amix where t is the transit time, D is the diameter of the pipe, and Amix is the speed of sound propagating through the fluid.
In the aforesaid homogenous “ideal” fluid flow, variation in transit time is analogous to a variation in sound speed of the fluid. In “real” fluid flows as diagrammatically depicted in FIG. 1, however, there are many mechanisms (e.g., entrained air bubbles, particles, vortices, etc., referred to hereinafter as “coherent vortical structures”) which: 1) convect with the fluid flow; 2) can cause small variations in normal incident ultrasonic signal transit time; and 3) remain spatially coherent for several pipe diameters. The term “spatially coherent for several pipe diameters” is used herein to refer to coherent vortical structures observed at one axial location which are subsequently observed to a degree at a downstream axial location after a period of time that is consistent with the time required for the structure to convect from the upstream to the downstream location. By monitoring the variations at two or more locations caused by the coherent vortical structures within the flow, the transit time of the variations between sensor locations can be processed to determine the velocity of the fluid flow.
To sense for such vortical structures, the ultrasonic transmitters (Tx) within an ultrasonic flow meter can be periodically pulsed to create the ultrasonic signal that transmits through the pipe and fluid. Each transmitter will have a fundamental oscillation frequency, which when pulsed will emit a short ultrasonic burst signal. In typical applications the receiver, located on the opposite side of a pipe, will receive this signal once it has bisected the pipe. However, in addition to this primary through-transmitted signal (i.e., the fluid borne signal component), other secondary signals will also be detected. These secondary signals include portions of the original signal that have been refracted or reflected along a different path through the pipe than the preferred direct transmission. Often these secondary signals possess sufficient strength to still reach the receiver and will interfere with the desired signal.
The dominant secondary signal is the “ring-around” signal. This is the portion of the ultrasonic signal that travels circumferentially through the wall of the pipe and can still be detected by the receiver. Because ring-around noise travels through the pipe wall rather than the fluid, it cannot provide any fluid flow information. FIG. 2 diagrammatically illustrates through-transmitted signals 10, scattered through-transmitted signals 12, and ring-around signals 14 traveling circumferentially within the pipe wall 16. Ring-around signals are created through reflection and diffraction between the transmitting ultrasonic transducer, the pipe wall and the material present inside the pipe typically due to the large impedance mismatch between the various materials. As an example, the impedance of steel such as, for example, in steel piping, is 45 MRayls in contrast to fluid which has an impedance of 1.5 MRayls. In this case, only a small percentage of the ultrasonic signal is actually injected into the fluid flow while the rest is reflected throughout the overall system. The majority of this excess energy is present in the pipe wall in the form of shear and compressional ultrasonic waves. If one considers the fact that the through-transmitted signal can be significantly attenuated as it travels through the fluid in the pipe, it becomes clear that it can be very difficult to distinguish the wanted signal from all the secondary signals.
It should be appreciated that the quality of any flow measurement, independent of the technology, is typically dependent upon the signal to noise (S/N) ratio. Noise, in this case, is defined as any portion of the measured signal that contains no fluid flow information. It is desirable to maximize the S/N ratio to obtain optimum performance.
Under ideal conditions, the ratio of the signal passing through the fluid to the ring-around noise is high (and/or the differential TOF between the signals is large) and it is relatively easy to produce the signal information required to make a flow measurement. On the contrary, in situations where the through-transmitted signal is significantly attenuated, and/or the ring-around signal arrives before the through-transmitted signal and is of the same temporal frequency, and/or the amplitude of the ring-around signal is significantly larger than the through-transmitted signal, the S/N ratio can be substantially reduced and the flow measurement compromised. In particular, the flow measurement can be compromised because of phase and amplitude errors introduced by the “noise” components of the received signal.
What is needed is a method for quantifying the modulating portions of the normal incident ultrasonic signals propagating through the fluid flow passing within the pipe, which method creates a favorable S/N ratio, and therefore an increased accuracy flow meter.